Multibeam Sonar Theory of Operation

Sidescan Sonar Multibeam Sonar Theory of Operation
Figure Chapter 5 - -3: Plot of Amplitude as a Function of Time
This simplified picture does not consider what happens to the transmitted pulse after it first strikes
the bottom, because a single-beam echo sounder is only interested in the time between
transmission and the earliest return echo. Yet to a sidescan sonar, the first returned echo only
marks when things start to get interesting. Look at what happens to the transmitted pulse as it
continues its spherical propagation.
Figure Chapter 5 - -4 shows the single-beam echo sounder schematic at a time shortly after the
ping first intersected with the bottom. At this time, the first return echo is on its way back to the
hydrophones. But the ping is still expanding, and in our schematic is intersecting the bottom at two
points. These points also create echoes. They are weaker than the first return (remember, as the
spherical pulse expands and propagates, it loses power), and they occur after it does (because it
took the ping longer to get to them). It is easy to see that as the pulse front continues to propagate,
it will produce a continuous series of weakening echoes in time. If the receiver continues to collect
these echoes, it will see a amplitude versus time sequence similar to the one pictured in Figure
Chapter 5 - -5, with a strong first bottom echo followed by a declining slope of continuous returns.
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